1. Field of the Invention
The present invention relates to an anisotropic conductive compound or material and a method of curing thereof.
2. Description of Related Art
Anisotropic conductive compounds are utilized to form conductive paths between pairs of aligned contacts, such as a contact on an integrated circuit or integrated circuit package and a contact of a printed circuit board. A typical anisotropic conductive compound includes conductive particles suspended in a binder. Such anisotropic conductive compound can be interposed in an uncured state between the integrated circuit or integrated circuit package and the substrate whereafter the anisotropic conductive compound can be cured to form conductive paths between contacts on the integrated circuit or integrated circuit package and the substrate while, at the same time, bonding the integrated circuit or integrated circuit package to the substrate.
Heretofore, anisotropic conductive compounds of the type described above were formed into a film which was interposed between the integrated circuit or integrated package and the substrate whereupon, with the application of pressure between the integrated circuit or integrated circuit package and the substrate in the presence of a curing heat, the conductive paths between aligned contacts of the integrated circuit or integrated circuit package and substrate, and the bonding of the integrated circuit or integrated circuit package to the substrate occurs.
A problem with prior art anisotropic conductive compounds is that they require the use of pressure and curing heat in order to form the conductive paths at the same time the bond is formed between the integrated circuit or integrated circuit package and the substrate. Another problem is that the film form of the prior art anisotropic conductive compounds requires special machinery in order to utilize the film in a production environment. Still, another problem is that the prior art anisotropic conductive compounds cannot effectively be utilized with integrated circuits, integrated circuit packages or substrates having adjacent contacts with edge-to-edge spacings less than about 300 xcexcm.
It is, therefore, an object of the present invention to overcome the above problems and others by providing an improved anisotropic conductive compound and a method of curing thereof. Still other objects will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.
Accordingly, I have invented anisotropic conductive material comprising electrically conducting material suspended in a binder. The electrically conducting material includes at least one of (i) nickel coated particles having a coating of noble metal, such as silver or gold, over the nickel coat and (ii) gold or silver coated nickel particles.
The particles having the nickel and gold, or silver coatings included at least two of carbon/graphite spheres, glass spheres and mica particles or flakes. The glass spheres can be solid or hollow. The gold or silver coated nickel particles can have a spherical shape, a flake-like shape or some combination thereof.
Where it is desired to form electrically isolated conductive paths between adjacent aligned pairs of contacts having an edge-to-edge spacing as close as 80 xcexcm, the particles including their coatings can have an average maximum dimension between 10 xcexcm and 25 xcexcm. For adjacent aligned pairs of contacts having a larger edge-to-edge spacing, the use of particles including their coatings having a larger average maximum dimension can be considered.
The binder is formed from the reaction product of a catalyst and a compound that includes an aromatic epoxy resin, a dimer fatty acid diglycidyl ester and an oxirane. The catalyst can include a quaternary cyanyl R-substituted amine. The aromatic epoxy resin can be formed from the reaction product of bisphenol-A and epichlorohydrin.
The binder can further include a UV curable modifier formed from the reaction product of a C1-C20 linear or branched alkyl (meth)acrylate, a (meth)acrylated urethane and a C1-C20 linear or branched hydroxy alkyl ketone. The binder can further include a phenolic resin. The phenolic resin may be a novalac resin formed as the reaction product of formaldehyde and one of phenol, cresol and bisphenol-A.
Alternatively, the binder can be formed from the reaction product of a novalac resin, a catalyst and one of a thermally polymerized aromatic epoxy resin and a phenoxy modified epoxy novalac resin.
The viscosity of the uncured anisotropic conductive material typically decreases with temperature. At 25xc2x0 C., the uncured anisotropic conductive material has a viscosity of at least 20,000 cps, in some cases at least 25,000 cps and in other cases at least 30,000 cps. Further the 25xc2x0 C. viscosity of the uncured anisotropic conductive material is up to 100,000 cps, in some cases up to 75,000 cps, in other cases up to 50,000 cps and in some instances up to 45,000 cps. The 25xc2x0 C. viscosity of the uncured anisotropic conductive material may vary between any of the viscosities recited above. Between 75xc2x0 C. and 150xc2x0 C., the viscosity of the uncured anisotropic conductive material is less than 10,000 cps, in many cases under 5,000 cps, in other cases less than 1,000 cps, in some instances less than 500 cps, in other instances less than 100 cps, and typically less than 50 cps. The viscosity of the anisotropic conductive material is measured utilizing a Brookfield Viscometer, such as Brookfield Viscometer model LVT, with a number 6 spindle at 10 RPM and 25xc2x0 C.
I have also invented a method of forming an electronic assembly. The method includes providing a substrate having a first arrangement of conductive contacts and providing an electronic component having a second arrangement of conductive contacts. An uncured anisotropic conductive material is deposited on the first arrangement of conductive contacts and the electronic component is positioned thereon with each conductive contact of the first arrangement aligned with a corresponding conductive contact of the second arrangement. The anisotropic conductive material is heated to a curing temperature for a curing interval sufficient to cause it to cure to a solid. During heating of the anisotropic conductive material, it is subjected to an AC magnetic field followed by a static, substantially homogeneous DC magnetic field. Each magnetic field has a field vector direction that is substantially parallel with the alignment of each conductive contact of the first arrangement with the corresponding contact of the second arrangement. The DC magnetic field can have a magnetic field strength between 400 and 1,500 gauss. The frequency of the AC magnetic field can be in the ultrasonic frequency range, namely, between about 20 kHz and about 500 kHz. The curing temperature and the curing interval can be between 70xc2x0 C. for about thirty minutes and 150xc2x0 C. for about 5 to 7 minutes.
The method can further include, after the electronic component positioned on the anisotropic conductive material and before the anisotropic conductive materials heated to its curing temperature, applying a UV curable adhesive between the substrate and the electronic component and exposing the UV curable adhesive to UV light to cause the UV curable adhesive to cure.